Free tool · plaster
How many bags of plaster?
I've spent over 20 years in UK construction, most of those years plastering and drylining, and I still work the bags out from the area and the thickness before I leave for the yard.
This page turns a wall or a ceiling into 25 kg bags of Thistle MultiFinish. You can add a Thistle BondingCoat or Thistle HardWall undercoat. It is a rough guide. The figures stay in the browser. Boards for the same job are the plasterboard calculator.
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What is on the sheet is on the Self Assessment checklist.
How the bags are worked out
The area is the length times the height, or the square metres you type, minus openings. The bag count is that area divided by the coverage on the data sheet, then increased by the wastage you type. Bags to buy are rounded up. Water is the litres per bag on the same sheet, times every bag you buy.
Thistle MultiFinish (opens in a new tab) is 10 m² per 25 kg bag at 2 mm, on walls and ceilings, with 11.5 litres of water a bag. Thistle BondingCoat (opens in a new tab) is 3.5 m² per 25 kg bag at 8 mm, and 2.75 m² at 11 mm, with 14 litres of water. Thistle HardWall (opens in a new tab) is 3 m² per 25 kg bag at 11 mm, with 15 litres of water. The White Book plaster systems (opens in a new tab) describe 11 mm of undercoat to walls, or up to 8 mm to ceilings, plus 2 mm of finish. Read on 18 October 2026.
At those printed thicknesses, the page uses the printed coverage. At any other thickness it scales the bags in proportion. BondingCoat scales from the 11 mm figure, except at 8 mm, where the sheet has its own number. That scaling is not a line on the data sheet. A blown wall is not a 2 mm skim. The measuring guide on this site says the same thing about a straight edge.
One wall or ceiling at a time. Add the bag counts yourself if the job has more than one. The 10% wastage is a starting figure, not a manufacturer allowance. Other free tools are on the tools page, and the measuring notes are in the measuring guide.
Common questions
Where do the bag figures come from?
British Gypsum's data sheets, read on 18 October 2026. Thistle MultiFinish is 10 square metres per 25 kg bag at 2 mm. Thistle BondingCoat is 3.5 square metres per 25 kg bag at 8 mm, and 2.75 at 11 mm. Thistle HardWall is 3 square metres per 25 kg bag at 11 mm. The links are under the calculator.
What if I skim thicker than 2 mm?
The MultiFinish sheet prints the coverage at 2 mm. This page scales the bags in proportion to the thickness you type. That scaling is not a second line on the sheet. The same is true for a HardWall thickness other than 11 mm, and for a BondingCoat thickness other than 8 mm or 11 mm, which is scaled from the 11 mm figure.
Does 10% wastage come from the data sheet?
No. 10% is a starting figure for drops, the tub, and a bit left in the bag. Change it. The bags to buy are rounded up either way.
Can I use this for a different plaster?
Only if that product's own coverage matches the one named here. A different brand, or Thistle BoardFinish, needs the figure on its own bag or data sheet. The line under the result says the same thing.